1. The Technical Field
This invention relates generally to fiber optics. More particularly, it relates to optical fiber couplers and methods for making such couplers.
2. The Prior Art
The explosion of demand for bandwidth has resulted in fiber optic communication cables containing multiple fiber ribbons, with each such ribbon typically including twelve or twenty-four fibers and 48-fiber ribbons being contemplated. Accordingly, there is a need for large numbers of ribbon couplers in central offices, branch offices and numerous other sites throughout the new high-bandwidth optical networks now being installed.
Devices and procedures for optically coupling single optical fibers are known in the art. For example, U.S. Pat. No. 4,626,652 for a “Method and Means of Removing Claddings from Optical Fibers” describes a process for using a laser to ablate the cladding from one side of an optical fiber to nearly expose the core region, as illustrated in FIG. 1. In this process, light is transmitted in the core of the fiber during the ablation step, while the “throughput” of the fiber is monitored. The ablation is stopped when the throughput changes by a predetermined amount to indicate that the core is nearly exposed. Two fibers prepared in this way are bent and placed with their flats together as shown in FIG. 2. The two fibers then are heated to near-melting and fused together, as shown in FIG. 3. During the fusing step, the two fibers are pulled (like taffy), necking them down until the desired power division is achieved.
Another procedure (not illustrated) for coupling a single pair of optical fibers, and one that represents common commercial practice involves: (1) removing the coating from a portion of each of two optical fibers; (2) twisting the uncoated fiber sections about each other; (3) heating the twisted section until the fibers soften; and (4) drawing the twisted glass structure axially to reduce the diameter thereof. Alternatively, the process involves placing the two stripped fibers inside a glass tube and then heating and drawing the composite structure. In either case, when the fiber diameters are sufficiently reduced, the evanescent fields of the two fibers overlap, and some portion of the light propagating in one fiber couples into the second fiber. During the drawing process, light can be propagated in one of the fibers, and the fraction of light coupled over to the other fiber can be monitored to control the process and produce couplers with specified coupling performance.
Single fiber coupler technology is not readily applied to multiple fiber applications, and particularly ribbon fiber applications, for several reasons. For one thing, the use of multiple single-fiber couplers on large numbers of ribbons is prohibitive in practice in that it requires a large amount of space, which is at a premium in most applications. Further, the foregoing processes of heating and drawing the two bare fibers together yield couplers which are very fragile and prone to damage, especially when used in congested environments. Also, simultaneously drawing all the fiber pairs in a pair of ribbon fibers is not likely to produce uniform couplers having the same coupling ratio because small variations in the core interaction are likely to occur from coupler to coupler.